The present invention relates to a method for controlling a steer-by-wire steering system.
Traditionally, in a land vehicle, the steering, that is to say all of the means that are used to direct the course followed by the vehicle, is produced using a mechanism which connects a control member, generally a steering wheel, to at least one steering axle generally comprising two steer wheels and which makes it possible to alter the orientation of the wheels with respect to the vehicle. The mechanism may be power-assisted by electrical or hydraulic means in order to facilitate the transmission of the force used to turn the control member up to each wheel of the steering axle.
There is also another type of steering system known as steer-by-wire steering in which the control member is mechanically decoupled from the steering axle.
Schematically speaking, a steer-by-wire steering system comprises a control member (steering wheel, hand tiller, joystick) on which a driver can act. By acting on the control member, the driver sets a direction to be given to the vehicle; the movement that the driver applies to the control member is identified by a position sensor (an angle sensor in the case where the control member is a steer wheel); the signal originating from the position sensor that senses the position of the control member is processed appropriately, then transmitted to one or more actuators which can act on the orientation of the wheels with respect to the vehicle, according to the action that the driver has exerted on the control member. It may be emphasized that a resistive torque is applied to the control member so that the driver has a feeling that reproduces the force exerted by the wheel on a running surface. This resistive torque may be a function of numerous factors; these may, for example, include the speed or acceleration of the vehicle, the speed or acceleration of the action exerted by the driver on the control member.
This type of steering has numerous advantages over conventional steering systems. This type of steering in particular reduces the risk of injury by contact with the steering column in the event of an accident and can be readily parameterized. It is also particularly advantageous in the case of industrial vehicles, of the truck type, which are bound by tight architectural constraints and generally have a cabin decoupled from the chassis. A steer-by-wire steering system in particular avoids having to resort to a conventional steering column, which has to be rigidly fixed both to the cabin and to the chassis.
Thanks to the electronic control exerted on the wheel orientation actuator or actuators, it becomes possible to use the steering as an element to make the running of a vehicle safer. It is thus possible, under particular driving circumstances, to give the steer wheels an orientation that may make it possible to maintain the stability of the vehicle and thus avoid a loss of control of the vehicle that could possibly lead to an accident.
One driving circumstance that potentially leads to accidents may be braking on an area where the adhesion is asymmetric; that is to say on an area that exhibits different coefficients of adhesion.
Typically, this type of circumstance is encountered when a vehicle is running along a road, which has a sheet of black ice or a puddle. In such a scenario, one wheel of the steering axle may find itself on a surface that exhibits good adhesion while the second wheel of the steering axle may find itself on a surface of low adhesion; insofar as these two wheels are subjected to the same braking action, the vehicle may then pivot, with respect to the wheel which is in contact with the surface of higher adhesion, and this may cause sideslip as the rear axle steps out. This is commonly known as “swapping ends” when the vehicle pivots through 180°. In other words, the yaw Rz, that is to say the moment of the force applied to the Z-axis of the vehicle, exceeds a value that the vehicle can withstand and the vehicle no longer follows the course set for it by the driver.
To combat this type of incident it is known, for example, in the case of a vehicle equipped with an antilock braking system (ABS), to regulate the braking power at the wheel in contact with the running surface having the lower coefficient of adhesion to prevent this wheel from locking up. That nonetheless has the negative consequence of an overall loss in braking power and a lengthening of the vehicle stopping distance because the braking system uses, as its reference, the wheel that is in contact with the surface of lower adhesion.
Another measure that makes it possible to combat sideslip in the case of braking on a surface that has asymmetric adhesion is to steer into the skid; this measure is aimed at orienting the steer wheels in the opposite direction to the direction of sideslip. In the case of steer-by-wire steering, it is possible in a sideslip situation to act on the steering actuator or actuators in order to steer into the skid even before the driver has reacted to the sideslip situation. It may be recalled on this subject that the reaction time of a driver is 0.7 second at best, whereas a steer-by-wire steering system can act on the wheels in a far shorter space of time.
A problem may nonetheless arise when the vehicle, the steer wheels of which have had opposite lock applied to them, leaves the area of asymmetric adhesion. What happens then is that the vehicle is in a condition in which its steer wheels are not along the axis of the vehicle although the conditions of adhesion are such that the two wheels are in contact with a surface exhibiting a uniform coefficient of adhesion.
Thus, it appears that the way of managing a vehicle that has to brake on a surface exhibiting asymmetric adhesion is not entirely satisfactory.
It is desirable that is to propose a method for controlling a steer-by-wire steering system that makes it possible to stabilize the path of a vehicle during a transition from an area of asymmetric adhesion to an area of uniform adhesion.
It is also desirable that is to propose a method for controlling a steer-by-wire steering system that makes it possible to stabilize the path of a vehicle in the braking phase during a transition from an area of asymmetric adhesion to an area of uniform adhesion.
According to an aspect of the present invention, a method is provided for controlling a steer-by-wire steering system of a vehicle having at least one steering axle having at least two steer wheels controlled by a control member. The method can comprise the steps of: detecting a difference in behaviour between a first steer wheel and a second steer wheel thereby indicating asymmetric steer wheel adhesion, the first steer wheel being in contact with a first area which exhibits a first coefficient of adhesion and the second steer wheel being in contact with a second area which exhibits a second coefficient of adhesion, the first coefficient of adhesion being higher than the second coefficient of adhesion; calculating a theoretical yaw parameter as a function at least of the actual speed of the vehicle and of the actual position of the control member; measuring the actual yaw parameter using a yaw sensor; comparing the actual yaw parameter with the theoretical yaw parameter; angling the wheel in contact with the area exhibiting the first coefficient of adhesion by an angle θ1, whereby the first steer wheel is oriented in a direction opposing the yaw, and angling the wheel in contact with the area exhibiting the second coefficient of adhesion by an angle θ2, with θ2 being comprised between a value whereby the second steer wheel is oriented substantially in the vehicle direction of travel and a value whereby the second steer wheel is oriented towards the first steer wheel, if the difference between the actual yaw parameter and the theoretical yaw exceeds a reference data.
The method according to the invention has the notable effect of not maintaining the near-parallel configuration of the steer wheels when these are steered into the skid in order to combat a yaw Rz applied about the Z-axis of the vehicle. In the case of asymmetric adhesion conditions, the method anticipates steering into the skid for the first steer wheel that is on the surface exhibiting the higher coefficient of adhesion—for example asphalt—so as to counter a moment applied to the Z-axis. At the same time, the second steer wheel which is on the surface having the lower coefficient of adhesion—for example a sheet of black ice or a puddle—remains in the vehicle direction of travel or is oriented towards the second steer wheel. In the latter scenario, the steer wheels are oriented in opposite and converging directions and adopt a “snowplough” position. Thus, when the vehicle leaves an area of asymmetric adhesion to regain an area of uniform adhesion, the behaviour of the vehicle is far more stable than when the vehicle regains an area of uniform adhesion with its two steer wheels in a position of steering into the skid. It may be noted that the fact of not steering into the skid, for the steer wheel that is in contact with the surface having the lower coefficient of adhesion, has no major impact on the action aimed at countering the yaw Rz. The action aimed at countering the yaw Rz is essentially due to the steer wheel that is in contact with the surface having the higher coefficient of adhesion; the action of the wheel in contact with the surface having the lower coefficient of adhesion is in any event limited.
In a possible embodiment, the method according to the invention can be implemented for controlling a steering system of a vehicle having at least one steering axle having at least two steer wheels; the steer-by-wire-type steering system can comprise, in particular: at least one sensor making it possible to identify the position of a control member; two actuators able to act respectively on each of the steer wheels in order to orient them independently by an angle θ1 and θ2 with respect to the longitudinal axis of the vehicle; a CPU able to receive an input signal originating from the sensor and able to emit an output signal bound for each of the actuators.
The method for controlling this steering system comprises the steps of: detecting a difference in behaviour between a first steer wheel and a second steer wheel thereby indicating asymmetric steer wheel adhesion, the first steer wheel being in contact with a first area which exhibits a first coefficient of adhesion and the second steer wheel being in contact with a second area which exhibits a second coefficient of adhesion, the first coefficient of adhesion being higher than the second coefficient of adhesion; calculating a theoretical yaw parameter as a function at least of the actual speed of the vehicle and of the actual position of the control member; measuring the actual yaw parameter using a yaw sensor; comparing the actual yaw parameter with the set-point yaw parameter; acting on the actuator of the first steer wheel in contact with the area exhibiting the first coefficient of adhesion in order to angle the steer wheel by an angle θ1 such that the first steer wheel is oriented in a direction opposing the yaw; and acting on the actuator of the second steer wheel in contact with the area exhibiting the second coefficient of adhesion in order to angle the wheel by an angle θ2, with θ2 being comprised between a value whereby the second steer wheel is oriented substantially in the vehicle direction of travel and a value whereby the second steer wheel is oriented towards the first steer wheel, if the difference between the actual yaw parameter and the theoretical yaw exceeds a reference data.
According to a favourite form of the invention, the step of detecting the difference in adhesion between the two steer wheels is performed while the vehicle is under braking, because it is under braking that the vehicle is most likely to escape the control of its driver.
In one implementation of the method according to the invention, the angle θ2 formed by the steer wheel in contact with the surface having the lower coefficient of adhesion and the longitudinal axis of the vehicle can be nil. This scenario can be especially advantageous when the vehicle runs on a straight line and brakes on an asymmetrical adhesion surface. The steer wheel which is in contact with the area having a high coefficient of adhesion is angled so as to combat the yaw but the steer wheel, which is in contact with an area exhibiting a low coefficient of adhesion, can remain in the longitudinal axis of the vehicle. This may be advantageous when the braking set-point is not very high, that is to say when the driver merely wishes to slow his vehicle down.
In another implementation of the invention, the angle θ2 formed by the steer wheel in contact with the surface having the lower coefficient of adhesion and the longitudinal axis of the vehicle is oriented in a direction equal to the direction of the yaw speed and is greater than zero. The vehicle is then in a configuration in which its steer wheels form a “snowplough” and this enhances the effectiveness of the braking while at the same time keeping the vehicle in a straight line when the vehicle regains conditions of uniform adhesion. This might be advantageous if the vehicle driver wants to bring his vehicle to a complete stop.
It can be preferentially anticipated for the angle θ1 formed by the steer wheel in contact with the surface having the higher coefficient of adhesion and the longitudinal axis of the vehicle and for the angle θ2 formed by the steer wheel in contact with the surface having the lower coefficient of adhesion and the longitudinal axis of the vehicle to be oriented in opposite directions and to have the same magnitude. This makes it possible to maintain symmetry in the orientation of the wheels and this is beneficial to the stability of the vehicle in the transition between an area of asymmetric adhesion and an area of symmetric adhesion.
In the case of a vehicle running along a curve, the steer wheels of the vehicle are respectively angled by θ1i and θ2i; θ1i and θ2i are substantially equal and are a function of the radius of the curve. Should a vehicle encounter asymmetrical adhesion conditions, the method may comprise the step of modifying the initial angles θ1i and θ2i formed respectively by the steer wheels with the longitudinal axis of the vehicle by respectively θ1 and θ2.
In one practical embodiment of the method according to the invention, the step of detecting a difference in behaviour thereby indicating asymmetric steer wheel adhesion, one wheel being in contact with an area having a first coefficient of adhesion and one wheel being in contact with a surface of low adhesion, is performed using the speed sensors of each of the steer wheels of an antilock braking (ABS) system.
These and other advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as a non-limiting example, an embodiment of a seat according to the invention.